The production of steel and iron creates various byproducts that contain zinc and lead, both relatively harmful metals. The particles stripped from the stack gases of such a plant contain some iron, and a portion of zinc and lead that depends on the type of additives, that is ore, sinter, pellets in a blast furnace, scrap for steel-making, and also on the process employed. The particles and slags from a blast furnace normally have a zinc content between 1% (by weight as hereinafter) and 40% and a lead content between 0.2% and 10%. In a steel plant the zinc content of the byproducts is between 0.2% and 5%, and occasionally higher, and the lead content between 0.1% and 2%.
Zinc, which is found in some ores and which is present when galvanized scrap is processed, is a particular problem in a blast furnace because of the low temperature at which it is vaporized and reduced. Thus any zinc that is not oxidized stays in the furnace where it builds up on and reacts with the walls. Lead, because of its very high vapor pressure, sinks in the furnace to form a pool underneath the hot metal in the hearth where it serves no useful function. Thus it is out of the question to reintroduce lead- and zinc-containing fines, even when they have a high carbon and/or iron content, back into the process. As a result these lead- and zinc-containing materials are simply stockpiled for lack of anything better to do with them. Since zinc and lead can be leached out of these particles and slags, such storage is a particularly difficult environmental problem. In addition such profitless storage or disposal of such materials means the loss of a substantial amount of iron, carbon, zinc, and lead. Obviously the current storage solution is a short-term one offering no long-term environmental protection.
For these reasons several different treatment methods have been tried which make it possible to separate zinc and lead out of such materials and to metallize the iron content in a solid form so that it can be fed back into the production line at the smelting plant.
Mainly the zinc- and lead-containing particles, which either already include some carbon-containing material to reduce the content of zinc and lead oxide or to which such carbon-containing reducing agents have been added, are mixed with an appropriate binder and are then transformed by pelletizing into small bodies with sufficient cohesiveness for handling and treatment. The thus obtained bodies are normally heated in a rotary kiln for sufficient time at 1000.degree. C. to 1200.degree. C. to vaporize the zinc. This reduces all of the zinc oxide to metallic zinc which passes as a vapor into the interior of the furnace and is carried off by a gas stream or the exhaust. Normally the zinc is oxidized in the furnace and the zinc oxide is separated as airborne particles from the gases. Lead is stripped out mainly as a chloride or a sulfide.
The hitherto known processes only allow a modestly efficient treatment if byproducts with a relatively high percentage of the materials to be removed can be used and if for efficiency's sake large amounts of the byproducts are available for continuous treatment. In addition the pellets thus produced are normally too small for further treatment. These pellets must therefore be briquetted in order to be used for example in a blast furnace. Such briquetting is expensive because their heat is lost before they are again reintroduced into the process.
These methods as well as the also known leaching methods do not solve the ecological problem. A large portion of the byproducts must be stored because it does not have a sufficient quantity of the materials for economically efficient treatment. Thus the carbon and iron contained in these byproducts is lost.